1. Field of the Invention
The present invention relates to a document reading apparatus which reads an image of a document, and an image processing method.
2. Description of the Related Art
A document reading apparatus (hereinafter, referred to as “image reading apparatus”) reads an image of a document, and converts information on the read image of the document into digital data. The image reading apparatus is capable of reading a monochrome image and a color image. The image reading apparatus is provided to an image forming apparatus such as a copying machine and a facsimile machine.
Conventionally, as the image reading apparatus to be used in a copying machine and the like, there has been known an image reading apparatus which performs so-called “flow-reading,” in which an auto document feeder feeds documents one by one onto a document glass plate and an image of each document is read at a fixed position on the document glass plate while being fed.
This type of image reading apparatus is provided with an illumination device which illuminates a document. The illumination device comprises a rare gas fluorescent lamp typified by a white xenon lamp, or an LED array as a light source. In general, illumination light intensity of those types of light source tends to gradually decrease with time since the start of illumination.
In the conventional image reading apparatus capable of performing “flow-reading,” when successive flow-reading is performed, a shading correction value is obtained before reading the first document, and thereafter the same correction value is used until the successive flow-reading is finished. Therefore, when the successive flow-reading is performed, an image quality is highly dependent on the illumination device, which greatly affects the document reading. In view of this, the technologies for avoiding and correcting deterioration of the light source have been improved.
In the image reading apparatus described in Japanese Patent Application Laid-Open No. 2007-081968, when the successive flow-reading is performed, there is obtained, for each inter-document spacing, an average value of image data corresponding to a predetermined number of pixels at each of several preset positions of a reference member (a conveyance belt) in a main scanning direction. By calculating a fluctuation amount of the average values of the respective positions, the light distribution fluctuation of the illuminator in the main scanning direction is corrected.
In the image reading apparatus described in Japanese Patent Application Laid-Open No. 2006-013852, there is calculated, for each inter-document spacing, an average value of image data corresponding to a predetermined number of pixels at each of several preset positions of a reference member (a guide member) in the main scanning direction. Among the average values of the respective positions, an average value exhibiting the smallest fluctuation amount is used for correcting the entire region in the main scanning direction. In this manner, when the correction value is obtained, erroneous correction due to a blot on the reference member is prevented.
In the case of the configuration of the image reading apparatus described in Japanese Patent Application Laid-Open No. 2007-081968, however, the reference member (conveyance belt) is normally arranged on a document path (a conveyance path), and hence the reference member is stained with paper dust, ink, toner, or the like of the document. For this reason, the reference member is stained with foreign substances, and as a result, a proper correction value (correction coefficient) cannot be obtained, which leads to a problem in that desired correction cannot be performed.
In the image reading apparatus described in Japanese Patent Application Laid-Open No. 2006-013852, an average value of image data corresponding to a predetermined number of pixels at each of several preset positions is calculated, and among the average values of the respective positions, an average value exhibiting the smallest fluctuation amount is used for correcting the entire main scanning region. Accordingly, it is possible to avoid the above-mentioned erroneous correction due to a blot on the reference member. However, even in a case where any one of the xenon lamp and the LED array is used as the light source, when a successive flow-reading operation is performed, the light intensity fluctuation amount differs among the main scanning positions because of, for example, a difference in degree of temperature rise among the main scanning positions. The correction described in Japanese Patent Application Laid-Open No. 2006-013852 is performed uniformly in the main scanning direction, and hence there is a problem in that proper correction cannot be performed when the fluctuation of the illuminator differs among the main scanning positions.